Vortigaunt

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Previously known as alien slaves, vortigaunts, or vorts, these are the Xen aliens first seen in Half-Life. They seem to have mastered electrical surges, and can charge themselves up to disperse energy, from great jolts of damaging current, to a steady flow that can recharge devices (like Gordon'ssuit).

In Half-Life 2 they have assimilated themselves into the suppressed society existant in City 17.

Quotes from HL2 suggests that the vortigaunts were controlled by the Nihilanth for eons much like Breen controlled the human race: The Nihilanth was a Combine product, and the vorts mostly rejoiced when it died, but it was only "the lesser master" and they now work together with Freeman to kill "the greater master".

The vortigaunt seem to refer to "a deeper mystery" and "something secret steers us both", which seems to refer to the G-Man, so they seem to have been contacted by the G-man too, or at least someone from his origin. The vorts have made the stand to fight against the Combine or be extinct: They think their lives are worthless unless spent on freedom.